Abstract
Earth’s human population continues to grow, and the demand for food and supplies is increasing as well. These increasing demands lead to a further intensification of agricultural activities, overgrazing, and deforestation. These, and other factors, contribute to the degradation of the physical structure and functionality of soil. In addition, climate
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change is intensifying extreme hydrological events, resulting in floods and more severe droughts. The proper functioning of soil is critical to sustaining the delivery of vital ecosystem services of both natural and agronomic ecosystems. Soil structure and function are highly dependent upon the stability of aggregates. The stability of soil aggregates refers to the ability to keep intact when exposed to different stresses. The research presented in this thesis seeks to examine the potential of microbial amendments as a strategy for improving soil structure and function under drought. We isolated and identified a collection of bacteria and fungi from a drought experimental field and utilized them in a series of experiments. In Chapter 2, a trait-based approach, relying on laboratory plate experiments, was used to select 24 bacterial strains that represented a range of predicted abilities to influence soil aggregations. These strains were inoculated individually in soil sterile microcosms under two moisture regimes (-0.03 and -0.96 MPa), considered optimal and close to permanent wilting points for plants, respectively. After 8 weeks of incubation, we found that bacteria improved aggregation better at high moisture, and bacterial traits provided a little predictive power to explain impacts on soil properties. Taxonomic affiliation had, however, a higher correlation to aggregation. In Chapter 3, we selected 29 fungal strains with higher abundance in drought field plots and taxonomy relevance to agriculture and inoculated them into soils using the same soil microcosm and moisture conditions as in the previous chapter. Fungal inoculation led to higher aggregate stability at both moisture regimes. This improvement of aggregate stability was explained by realized high fungal biomass and low sorptivity under drought. Fungal inoculation also showed higher soil water potential when compared to the control for the soil at high moisture after an event of drying. In Chapter 4, a selection of bacteria and fungi strains were inoculated in tomato plants, and we examined aggregate stability and plant growth under well-watered and drought regimes. The results showed that microbial inoculation improved soil aggregate stability at both moisture levels. Chlorophyll content, fresh shoot biomass, and soil water content were modified by inoculation under drought, and dry shoot biomass was affected at high moisture. However, no clear correlation was found between soil structure improvements and plant growth. In Chapter 5, we showed how microbes mediate plant ecological and evolutionary responses to global changes and how plant responses mediate eco-evolutionary feedbacks. We conclude that fungal inoculation showed a higher potential for improving soil aggregate stability than bacterial under both moisture levels. Bacterial and fungal inoculation improved plant growth at high moisture, and, even though a connection between soil aggregation and plant growth was not identified, we propose that additional research on the improvement of aggregates as a means of physical plant environment under drought is warranted. Long-term experiments, longer periods of microbial incubation, and the designing of multispecies communities may represent important steps forward. Although far from being completely understood, plant-microbe and microbe-soil interactions have the potential to play an important role in plant and soil adaptation in the face of global changes.
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